If you want the container to use the exim set up on the hosted computer, I’m pretty sure you use:
SMTP server address? $container_host_fqdn
SMTP user name? “”
SMTP port? 587
SMTP password? “”
That is, you only need the host name and port, not the other two values. Port 25 would be my old-fashioned choice for a local connection, but 587 might be prefered by Discourse.